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One of the main theories behind colon cleansing is an ancient belief called the theory of autointoxication. This is the belief that undigested meat and other foods cause mucus buildup in the colon. This buildup produces toxins, the theory goes, which enter the blood's circulation, poisoning the body.

Some people claim these toxins cause a wide range of symptoms, such as:
* fatigue
* headache
* weight gain
* low energy

On the surface, the idea of toxins being reabsorbed by the body makes some sense. After all, rectal suppositories are used to rapidly administer drugs. Could toxins, too, be quickly entering the bloodstream from the colon?
If colon cleansing has any effect, however, it likely has less to do with toxins than with its impact on the nervous system. What's the evidence? Reflexes in the bowel affect the entire nervous system. In fact, a wide range of symptoms caused by constipation can be relieved by enemas. If this is true of enemas, then perhaps colon cleansing has a similar effect.

http://www.captaincolon.com/Colon_FAQ.html